In a simple soil profile, which layer (horizon) is the topsoil rich in humus?

In a soil profile, the A-horizon is the uppermost mineral layer, commonly called the topsoil. It contains humus mixed with mineral particles, which gives it a dark colour and makes it the most fertile and biologically active layer.

What is a soil profile?

A soil profile is a vertical section of the soil, from the surface down to the parent rock. It shows a series of layers, called horizons, which differ in colour, texture, structure and composition.

Main soil horizons (from top to bottom):

O-horizon (organic layer): in many soils, especially under forests, a thin layer of fresh and partly decayed leaves, twigs and other organic litter lies on the surface.

A-horizon (topsoil): mineral soil mixed with humus. It contains most of the plant roots, earthworms, insects, bacteria and fungi. It is dark in colour and loose. In many soils, nutrients and fine particles are washed down from this layer by percolating water (eluviation).

B-horizon (subsoil): lies below the A-horizon. It contains less humus and receives clay and other minerals washed down from above (illuviation, or zone of accumulation). It is usually denser and lighter in colour than the topsoil.

C-horizon (parent material): partly weathered rock fragments from which the soil above has developed. It contains little or no organic matter.

R-horizon (bedrock): the solid, unweathered rock below.

Some classifications also recognise an E-horizon (a pale, leached layer) between the A and B horizons in certain soils.

Factors of soil formation:

Parent rock, climate, relief (slope), organisms (plants, animals and microbes) and time. Climate is considered the most important active factor.

Why topsoil is important:

Most crop roots grow in the topsoil, which holds most of the nutrients and organic matter.

It takes hundreds of years to form a few centimetres of topsoil, but it can be lost quickly by erosion. This is why soil conservation is so important.

Why not the other options?

• ✗The B-horizon (subsoil) is the zone of accumulation, with less humus.
• ✗The C-horizon consists of weathered parent rock.
• ✗The R-horizon is the unweathered bedrock.